This invention relates to a vertical medium-stirring type pulverizer provided with means for preventing wear of the inner surface of the pulverizing shell and the top surface of a screw blade.
FIG. 1 shows a vertical medium stirring type pulverizer which comprises a vertical pulverizing shell 1 filled with pulverizing medium b such as steel balls and a vertical screw shaft 2 mounted in the shell. With the screw shaft 2 in rotation, the material a to be pulverized is introduced into the shell 1 and is stirred up and down together with the pulverizing medium b. The particles of the material a collide with each other and with the pulverizing medium D, so that the material is pulverized into an end product c having a fine particle size. The end product c thus made is carried on the flow of a fluid d such as air or water and taken out of the shell 1.
Since the material a and the pulverizing medium b are stirred in the shell 1, the inner surface of the shell 1 and the surface of screw blade 5 are subjected to a large frictional force. Thus, it is necessary to provide some wear-preventive means on these portions. Typically, such a wear-preventive means was a wear-resistant liner laminated on these portions.
But liners having high wear resistance are usually expensive. On the other hand, cheap liners are less wear-resistant and thus have to be replaced frequently.
In order to prevent these problems, the present applicant has proposed a wear-preventive structure in Japanese Unexamined Utility Model Publication 4-37541. As shown in FIG. 9, this structure comprises ribs 4 provided on the top surface of the screw blade 3 of the screw shaft 2 and extending diametrically so as to be angularly, spaced apart from each other. This arrangement can reduce the wear of the screw blade 3.
While the material a and medium b are moving in the shell 1, the ribs 4 serve to prevent the movement of the medium and material on the blade 3. In this state, although the medium trapped on the blade 3 may be rubbed against the moving medium, no large frictional force will be produced between the trapped medium and the top surface of the blade 3. Namely, the trapped pulverizing medium performs the function of lining the screw blade. Thus, the moving pulverizing medium b never rubs the top surface of the blade 3, so that the blade 3 is protected against wear and the pulverizer can be operated continuously for a longer period of time.
But, as shown in FIG. 9, since the ribs 4 are erected, like walls, on the blade 3 at a right angle with respect to the direction of rotation (shown by arrow) of the screw shaft 2, they will reduce the force tending to raise the pulverizing medium b, produced by the blade 3, thus lowering the flowability. Also, as shown in the same figure, a cavity 5 tends to be formed behind each rib 4. No medium-raising force is produced in these cavities 5. Thus, the flowability will decrease.
Japanese Examined Patent Publication 44-29838 discloses a conventional means for forming a self-lining layer A (FIG. 10), made up of pulverizing medium similar to the self-lining disclosed in the above prior art, on the inner surface of the pulverizing shell. In this prior art, as shown in FIG. 10, vertical ribs (rods 15) and circumferentially extending ribs (ledges 14) are provided on the inner surface of the shell 1. Pulverizing medium b trapped in tile spaces defined by the ribs 14 and 15 performs the self-lining function.
But, in this arrangement, the pulverizing medium b tends to move downwards by gravity. Thus, the self-lining layer A in each space tends to be thinner at the upper portion than at the lower portion as shown in FIG. 1. Also, as shown in FIG. 12, the self-lining layer A tends to be thinner at the front portion with respect to the direction of rotation of the screw shaft 2. Thus, in order to form a self-lining layer A that covers the entire inner surface of the pulverizing shell 1, it would be necessary to reduce the distances between ribs 15 and 14 (rods and ledges). For this purpose, a greater number of ribs 14 and 15 have to be used and thus it is more troublesome to mount them. Moreover, the use of a greater number of ribs leads to an increased number of damaged ribs and thus more frequent replacement work. These factors will all push up the cost.
On the other hand, in horizontal medium stirring type pulverizers, a horizontal pulverizing shell is rotated by its shaft. A wear-preventive means for preventing wear on the inner surface of this type of pulverizing shell is disclosed in Japanese Examined Patent Publication 58-5706, U.S. Pat. No. 3,913,851, and U.S. Pat. No. 4,340,616, etc. This wear-preventive means is magnets stuck on the inner surface of the pulverizing shell. The magnets serve to attract magnetic pulverizing medium and the material to be pulverized, thus forming a self-lining layer A on the inner surface of the pulverizing shell.
But, with a horizontal pulverizing machine, the pulverizing medium tends to be repeatedly attracted to and separated from the magnets while the pulverizing shell is rotating. Thus, the self-lining layer A cannot be formed effectively.